Gas concentration is conventionally measured by means of the absorption of infrared radiation. The measurement techniques are categorized into dispersive and nondispersive IR methods (NDIR, nondispersive infrared). The dispersive method is based on the use of a grating or a prism for measurements on multiple wavelengths. The nondispersive method employs a wider spectral range of the infrared range. Nondispersive infrared analyzers have a simpler construction which is easier to use. The wavelength range of measurement is conventionally selected by means of a bandpass filter.
A measurement system based on NDIR techniques usually comprises the following parts: an IR radiant source, a measurement channel, a bandpass filter and an IR detector. The method has generally been sufficiently selective with respect to the material being measured, while the instability of the analyzer equipment has posed a problem. Such instability is related to the aging of the IR radiant source, contamination of the measurement channel and sensitivity changes of the IR detector. To overcome such an instability problem, a measurement method has been used in which the measurement is performed at two different wavelength bands. One of the bands is selected to coincide with the absorption lines of the gas being measured while the other band (reference band) is set aside from the absorption band. Here, two separate bandpass filters are required. An alternative approach to the construction of the measurement apparatus is either to mount a pair of mechanically changeable filters in front of the IR detector, or alternatively, use two fixed filters combined with two separate IR detectors. The changing mechanism for the filter pair is usually implemented by means of a rotating disk.
The disadvantages of the above-described technique include a complicated construction of the mechanical structures of the measurement apparatus and, resultingly, a relatively high cost of the implementation of the measurement apparatus. The bandpass filter conventionally employed in the measurement apparatus is a special component carrying a rather high price. The rotating disk is hampered in use by its rather short life cycle due to the wear of its bearings. Furthermore, the contamination of the separate bandpass filters occurs at different rates causing an error in the measurement. The use of two separate fixed bandpass filters and IR detectors is hampered by the different aging rates of the IR detectors and the different contamination rates of the bandpass filters.
K. Aratani et al., "Surface Micromachined Tuneable Interferometer Array," The 7th International Conference on Solid-State Sensors and Actuators, Transducers '93, 678, Yokohama 1993, discloses an electrostatically tuneable Fabry-Perot interferometer component manufactured by surface micromechanical techniques. The component includes an integral photodiode acting as the light detector that has been implemented by processing a p-n junction into the silicon substrate. The components are fabricated into an interferometer array in which the size of each component is only 20.times.20 .mu.m. The application of the array is to function as a light modulator in optical data transmission. Because a silicon diode is used in the component as the light detector, its use at wavelengths longer than approx. 1.1 .mu.m is impossible, whereby the use of this component in concentration measurements is essentially limited. The application of the component is also restricted by its small area making the output signal of the light detector excessively weak for use as a component of an NDIR measurement apparatus.
T. W. Kenny et al., "Novel Infrared Detector Based on a Tunneling Displacement Transducer," J. Vac. Sci. Technol. A10(4) (1992), discloses a silicon micromechanically fabricated infrared detector operating on the principle of the so-called Golay cell in which the detection of infrared radiation is based on the measurement of thermal expansion of a gas in a sealed chamber. The infrared radiation is absorbed in the wall of the chamber formed by a thin membrane with an extremely low thermal mass. Resultingly, heat is transferred from the membrane to the gas in the sealed chamber, whereby the temperature increase of the gas tends to expand the gas thus causing a movement of the membrane. The displacement of the membrane is measured from the tunneling current between a thin, pointed electrode manufactured by silicon micromechanical techniques and the membrane. Such a component acts as a wideband optical IR detector, and its use in concentration measurements would require a separate bandpass filter.
S. Bauer et al., "Thin Metal Films as Absorbers for Infrared Sensors," Sensors and Actuators A, 37-38, (1993), pp. 497-501, discusses the use of thin metal foils as absorption materials of thermal IR detectors. A freely suspended thin metal film can maximally absorb 50% of the IR radiation impinging on it. However, significant improvement of absorption can be achieved by using a substrate material of suitable refractive index which is coated by a metallic film. Then, the magnitude of absorption becomes strongly dependent on the wavelength of the IR radiation. However, Bauer describes no method of limiting absorption to a given wavelength range. Therefore, also this embodiment requires the use of a separate bandpass filter in concentration measurements.
Micromechanical techniques have been widely applied to the manufacture of thermal IR detectors. Such techniques are based on thermally insulating the active, IR radiation absorbing part of the detector from the silicon substrate of the detector. The temperature change of the active part can be measured by means of a thermocouple integrated in the structure, a pyroelectric material, or a temperature-sensitive resistor. All structures known in the art employ wideband optical detectors.